Testing as an art (Part 2)

The concept that any manufactured product should be tested was introduced in the first instalment of this series. Of course, a key aspect to testing is controlling the test. There are numerous approaches to creating the circuitry required to test any board. It is vital to remember that the test equipmentdocumentation is as important as the product itself. The product life cycle can extend beyond expectations. Test jigs are shipped back and forth to subcontractors, stacked on shelves, and abused in many other ways (sometimes intentionally), so they have to be rugged.

Quick and dirtyIt is sometimes possible to make your test equipment very quickly and cheaply, as you can see in figure 1.

Figure 1: Comparators and a simple PLD sequence generator with hexadecimal readout of failure codes, hand-wired on protoboard, form the heart of this test equipment.

This approach has some problems as well. The second unit takes just as long to build as the first and is rather fragile. Nevertheless this piece of test equipment has worked for 18 years with only one solder connection breaking.

Design your ownSometimes there is just no commercial equipment available to meet your unique requirements and you simply have to design your own. For example, we required a method of generating a stable 100 amps AC, preferably without high voltage or heat. Of course, creating the equipment can be a huge project in its own right. The problem with this approach is that it drains corporate resources and—if the project needs multiple iterations—you may have to compromise on the requirements.

Figure 2: This device, together with a user interface on a PC, will generate up to 100 amps AC. The current is controlled by a closed loop system implemented using an 8052.

Self–testIf you have a microcomputer on board, and provided you have sufficient memory, the whole test can be implemented as an integral portion of the firmware.

Figure 3: On the unit shown here, the test process is invoked by a jumper. When an input is activated a corresponding output activates. One problem with this test is that diagnosing where the fault lies is fairly coarse. (It's somewhere on the input or output path.)